The present invention relates generally to drilling and completion techniques for downhole wells, and more particularly to methods and apparatus for limiting the rotation of cementing plugs being drilled out of the plugs.
In the construction of oil and gas wells, a wellbore is drilled into one or more subterranean formations or zones containing oil and/or gas to be produced. During a wellbore drilling operation, drilling fluid (also called drilling mud) is circulated through the wellbore by pumping it down the drill string, through a drill bit connected thereto and upwardly back to the surface to the annulus between the walls of the wellbore and the drill string. The circulation of the drilling fluid functions to lubricate the drill bit, remove cuttings from the wellbore as they are produced and to exert hydrostatic pressure on pressurized fluid contained formations penetrated by the wellbore whereby blowouts are prevented.
In most instances, after the wellbore is drilled, the drill string is removed and a casing string is run into the wellbore while maintaining sufficient drilling fluid in the wellbore to prevent blowouts. The term xe2x80x9ccasing stringxe2x80x9d is used herein to mean any string of pipe which is lowered into and cemented in a wellbore including but not limited to surface casing, liners and the like.
Typically, at the beginning of a cementing job, the casing and hole are filled with drilling mud. Very often, a bottom cementing plug is pumped ahead of the cement slurry to reduce contamination at the interface between the mud and cement. The bottom plug is typically constructed to have elastomeric wipers to wipe the casing of drilling mud and thereby separate the drilling mud ahead of the bottom plug from the cement slurry behind the bottom plug. The casing string will have a landing platform for the bottom plug. The landing platform may be a float collar, a float shoe or a shoulder in the casing string. When the bottom plug seats upon the landing platform, the fluid pressure differential created across the bottom plug ruptures a diaphragm at the top of the bottom plug and allows the cement slurry to proceed down the casing through the plug, through the float equipment at the lower end of the casing and up the annular space between the casing and the wellbore.
Once the required amount of cement has been displaced into the well, a top cementing plug, which will likewise have wipers thereon, may be displaced into the casing. The top cementing plug will follow the cement slurry into the casing, and is designed to reduce the possibility of any contamination or channeling of the cement slurry with drilling fluid or other fluid that is used to displace the cement column down into the casing and into the annular space between the casing and the wellbore. The top cementing plug does not have a fluid passage therethrough such that when it reaches the bottom cementing plug, the top cementing plug will cause a shut off of fluids being pumped through the casing.
Once the cement has set up and any other desired operations have been performed, the cementing plugs, along with float equipment therebelow, may be drilled out. In order to do so, the drill string with the drill bit thereon is lowered into the hole until the drill engages the top plug and is rotated. In many instances, however, when the drill bit is rotated, the top plug also begins to rotate on top of the bottom plug, or the bottom plug may rotate on the landing platform, whether the platform is float equipment or a shoulder or other restriction in the casing. Plug rotation costs valuable time and therefore has an economic impact on the cost of the well. Thus, there is a need to eliminate or at least limit the rotation of the cementing plugs during drillout after the cementing job. Several attempts have been made at limiting the rotation of the cementing plugs. One such attempt is described in International Application No. PCT/US00/40545, International Publication No. WO 01/09481 A1, entitled Anti-Rotation Device for Use with Well Tools. Another device for limiting the rotation of plugs is described in U.S. Pat. No. 5,095,980, which discloses a combination non-rotating plug set. Other devices and/or methods are shown in U.S. Pat. No. 5,390,736, U.S. Pat. No. 5,165,474 and U.S. Pat. No. 4,190,111. Although the apparatus and methods described therein may in some cases work well to limit rotation of cementing plugs during drillout, there is a continuing need for an anti-rotation apparatus and method which will consistently limit the rotation of the cementing plugs during drillout and which is easy to use, efficient and inexpensive.
The present invention provides an apparatus for preventing, or at least limiting the rotation of a cementing plug during drillout of the cementing plug. The apparatus includes an outer case, which preferably is a joint of casing. The outer case may be referred to as an outer housing or outer sleeve. An inner sleeve is disposed in the outer case. The inner sleeve has an open upper end and an open lower end and is adapted to receive cementing plugs displaced through a casing string during a cementing job. The inner surface of the sleeve is configured and dimensioned so as to cause an interference fit, and thus frictionally engage cementing plugs that are received therein. Engagement between the cementing plugs and the inner sleeve will prevent or at least limit rotation of the cementing plugs during drillout of the cementing plugs after a cementing job. The inner sleeves are preferably comprised of a durable, drillable material.
In one embodiment, the inner sleeve has a tapered inner surface. The tapered inner surface preferably tapers radially inwardly from the upper end of the inner sleeve to the lower end of the inner sleeve. The tapered inner surface may have a circular cross-section so that the inner surface has a generally frustoconical shape, or may define a polygonal cross-section, so that the inner surface defines a polyhedral shape. The apparatus of the present invention limits rotation of cementing plugs by engaging the plugs that are received therein so that when rotational drilling forces are applied, rotation of the plug is prevented or is at least limited.